Membrane Distillation (MD) refers to a thermally driven transport of vapor through non-wetted porous hydrophobic membranes with the driving force being the vapor pressure difference between the two sides of the membrane pores. MD may be used for various applications including desalination, environmental cleanup (e.g., removal of volatile organic chemicals from water), and food and medical applications. Known MD configurations include direct contact membrane distillation (DCMD), sweeping gas membrane distillation (SGMD), vacuum membrane distillation (VMD), and air gap membrane distillation (AGMD).
Using MD processes for desalination has several advantages compared to other membrane-based desalination processes, such as reverse osmosis (RO). These advantages include the fitness for intermittent energy supply in an MD process, the membrane tolerance for occasional dry-out, the ability to use the process with direct solar heat without heat storage, and the possibility of using MD in small-scale individual units. Although research has been conducted on water desalination using MD processes, particularly in the VMD configuration, is currently still in the laboratory R&D phase, MD has not yet become commercially viable for desalination. One reason for non-commercialization is the high cost of water produced using this process (e.g., estimated at $5-20 per cubic meter of produced water), which makes it non-competitive compared to other desalination processes, such as RO.
In one type of VMD system 100, as shown in FIG. 1, a VMD module 110 separates water vapor and brine from salt water pumped from a salt water feed tank 112. The salt water is pumped by a feed pump 114 and heated by a water heater 116. A vacuum pump 120 generates vacuum pressure to draw the vapor out of a permeate chamber in the VMD module 110. The water vapor then passes through a condenser 122 to condensate and recover the water. A liquid or moisture trap 124 may be used to protect the vacuum pump 120.
There are several problems with this design. The power cost for the vacuum pump is relatively high. Also, recovery of water is performed by condensing the vapor using a heat exchanger, and the cost of running a coolant in the heat exchanger adds to the cost of produced water. Further, the recovery of the evacuated vapor in the condenser may be incomplete, unless condensation is done at a very low temperature with a significant energy consumption. An incomplete recovery of evacuated vapor means that a portion of the targeted product (i.e., water vapor) is lost and un-utilized. As a result, the overall cost of production, per amount of water produced, increases.
The incomplete recovery of vapor may also lead to vapor penetrating the vacuum pump. Vacuum pumps should be sealed and lubricated, and both the seals and lubricant are negatively affected by the entrance of vapor into the pump. Despite the attempt to use a trap to protect the pump, minute amounts of vapor will inevitably damage the vacuum pump on the long run.